The proposed research addresses the following specific questions: 1. do the muscles of respiration ever become fatigued? 2. if so, what is the prevalence of respiratory muscle fatigue in disease? 3. what factors either contribute to, or retarded the onset of, this phenomenon? 4. would it be useful and practical to develop a rapid monitoring device to detect the onset of fatigue in patients in a critical care unit? A technique is described for detecting the electrophysiologic manifestations of muscular fatigue, based on power density spectral analysis of respiratory electromyograms. The method utilizes microprocessor technology, with original software developed in this laboratory. Numeric indices are defined which, in preliminary studies, changed significantly during a fatiguing inspiratory maneuver. The first step of the proposed research will be to complete the validation of the technique by comparing these parameters with direct measurements of respiratory muscule pressures. Thereafter, the analysis will be applied, in human and animal models, to identify factors which are significant in the development of fatigue. The ultimate aim of the research is to establish a rapid-response monitoring system that can be applied to patient care. Concrete steps for achieving this goal are outlined.